Scintillation detectors have been employed in the oil and gas industry for well logging. These detectors have used thallium activated sodium iodide crystals that are effective in detecting gamma ray radiations. The crystals are enclosed in tubes or casings to form a crystal package. The crystal package has an optical window at one end of the casing, which permits scintillation light induced by radiation to pass out of the crystal package for measurement by a light sensing device such as a photomultiplier tube coupled to the crystal package. The photomultiplier tube converts the light photons emitted from the crystal into electrical pulses that are shaped and digitized by associated electronics. Therefore, the fraction of the induced scintillation light collected by the photomultiplier should be as large as possible. One reason, which leads to reduce this fraction of light, is the losses at the crystal surface. The uniformity of light collection depends primarily on the conditions, which exist at the interface between the crystal and the enclosing tube. In order to recapture the light that escapes from the surface, the crystal is normally surrounded by a reflector at all surfaces except that at which the photomultiplier tube is mounted. Two types of reflector can be used: a polished metallic surface acting as a specular reflector or a dry powder packed around surfaces of the crystal acting as a diffuse reflector.
The ability to detect gamma rays makes it possible to analyze rock strata surrounding the borehole, as by measuring the gamma rays coming from naturally occurring radioisotopes in downhole shales which bound hydrocarbon reservoirs. A common practice is to make those measurements while drilling (MWD); and for this type of application, the detector must be capable of withstanding high temperatures and also must have high shock resistance. At the same time, there is a need to maintain performance specifications.
A problem associated with MWD applications is that the reflector used in the detector, specular or diffuse, will suffer from those high temperatures and high shocks. When a diffuse reflector as a dry aluminum powder is used, after some shocks it will clog at one end of the crystal package and fail to reflect light at all surfaces. And when a specular reflector as polytetrafluoroethene (PTFE) tape, or any type of foil is used, the inhomogeneities of the crystal edge do not ensure a perfect contact between the crystal and the foil. Air and gas are irremediably trapped in the crystal/foil interface and it does not permit to the foil to act as a perfect specular reflector, reducing a large fraction of light induced by the crystal.
Another problem associated with MWD applications, is that detectors report a higher than an actual count rate if the scintillation crystal package produces vibration induced light pulses. The harsh shock and vibration conditions the detectors encounter during drilling can cause a crystal package to emit spurious light pulses in addition to gamma ray induced light pulses. That is, the detector output will be composed of radiation induced counts and vibration induced counts. Heretofore, the detector electronics could not distinguish the vibration induced counts from the genuine gamma counts, whereby the detector reports a higher than actual count rate. Some prior art solutions use electronic devices to filter out vibration induced counts by discriminating on the basis of the pulse shape and/or the signal decay time. Other solutions, directly applied to the package of the crystal, described in U.S. Pat. No. 5,869,836, use an elastomeric material which absorbs shocks and vibration. Commonly, this elastomeric material is associated with a PTFE tape to act as a reflector. This shock absorbing member solves the problem related to high temperatures and high shock resistance, but not to the problem of specular reflector. The present disclosure provides a solution to the aforesaid problem of reflector in the crystal package.